Field displacement isolator



May 13, 1958 QB ETAL 2,834,945

FIELD DISPLACEMENT ISOLATOR 3 Sheets-Sheet 1 Filed April 25, 1955 H.BOVET WVENTORS' s. WE/SBAUM QWWW ATTORNEY May 13, 1958 Filed April 25,1955 LOSS IN 08 L085 IN 08 H. BOYET ET AL 2,834,945 FIELD DISPLACEMENTISOLATOR 3 Sheets-Sheet 2 FIG. 4

FIG. 6

RETURN 1.085 f REVERSE L085 l l l l l l FREQUENCY IN MEGACVCLES PERSECOND FIG. 7

/ FORWARD Losy l l l FREOUENCE IN MEGACYCLES PER SECOND INVENTORS BOVETs. WE/SBA UM B Y.

A T TORNE Y y 3, 1958 H. BOYET ETAL 2,834,945

FIELD DISPLACEMENT ISOLATOR Filed April 25. 1955 3 Sheets-Sheet 3 FIG. 6

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INVENTORS' B-OYET $.WE/SBAUM BW ATTORNEY United States Patent FIELDDISPLACEMENT ISOLATOR Application April 25, 1955, Serial No. 503,382

6 Claims. (Cl. 333-451) This invention relates to nonreciprocaltransmission devices for use in wave guide systems and more particularlyto an isolator of the field displacement type.

An isolator is defined as a device which may be employed to isolate anelectromagnetic device from other portions of an electromagnetic wavesystem, in the sense that waves may be freely transmitted in thedirection from the device through the isolator to the system, designatedthe forward direction, but waves originating outside of the device andtraveling in the opposite direction, desig hated the reverse direction,are attenuated by the isolator to the extent required to preventdeleterious reacion of the system upon the device to be isolated.

An isolator of the field displacement type is one which operates byvirtue of a nonreciprocal action whereby the field pattern in a waveguide is modified or displaced so that the field pattern is materiallydifferent depending upon the direction of propagation of the wavethrough the isolator.

In accordance with the invention, there are employed two elements ofgyromagnetic material, such as ferrite,

each having two parallel fiat faces defining a thickness therebetweenwhich is moderately critical in value relatively to the free spacewavelength of a given wave to be transmitted. The said elements maybeplaced Within a wave guide of rectangular cross section with the saidfaces parallel to the narrower side walls at a critical spacingtherefrom, whereat the field pattern of wave transmission for onedirection of propagation has substantially a null value of the electricfield component at one face of each of the gyromagnetic elements.

To minimizereflections due to the presence of the gyromagnetic materialthe latter preferably, does not extend the full distance between thelonger sides of the rectangular wave guide, i. e., it is less inheightthan the narrower dimension of thewave guide. The intensity of thereflected wave returning is thereby rendered small compared to the waveimpressed upon the isolator, a condition commonly designated as a largereturn loss.

. Resistive material for producing loss in the reverse direction isplaced upon the said face in the null region with the effect thatsubstantially no attenuation results for awave propagated in the forwarddirection. The

gyromagnetic' material reacts differently to waves propagated intherespective directions, with the result that a high intensity of theelectric field component is developed at the location of the resistivematerial for a wave propagated in the reverse direction.

Letting represent the thickness of the gyromagnetic element between theparallel faces, 7 the spacing between one face and the nearestnarrowside wall of a rectangularwave guide of longer side dimension Land .t the thickness of the resistive material (applied as a sheet orfilm upon the second face of the gyromagnetic material), and t the freespace wavelength at the center of a desired frequency band, the bestspacingis found to be determined by the formula Patented May 13, 1958"ice equal to a constant is found to hold, for 6 in the range of 0.175to 0.200 inch. In general, the divisor of A difiers from 9.55 if theresistive coating is a sprayedon film instead of an attached sheet.

For broad band operation, the resistive material should be arranged in aparticular configuration. Where the gyrornagnetic element is less inheight than the narrower dimension of the wave guide and the element hasa rectangular face upon which the resistive material is placed, ithasvbeen found that there are certain portions of the face where maximumabsorption in the resistive material occurs at the lower frequency sideof a broad frequency band and other portions where maximum absorptionoccurs at the higher frequency side of the band. For the lowerfrequencies the region of maximum absorption is a centrally locatedlongitudinal strip approximately one-half the length of the longer sideof the rectangular face. For the higher frequencies there are tworegions of maximum absorption in diagonally opposite corners of therectangular face. All of these regions may be covered sufiiciently in aconvenient form by means of a centrally located longitudinal strip ofresistive material and a diagonally located strip approximately thelength of the diagonal of the rectangular face. Which of the twopossible diagonal positions the latter strip occupies has been found notto be important.

An object of the invention is to obtain a large ratio of reverseloss toforward loss over a relatively broad frequency band, andmoreparticularly a large reverse loss coupled with a small forward loss.

Another object is to maintain a relatively large return loss over theentire frequency band.

As herein used and in accordance with common terminology, reverse lossis the attenuation suffered by a Wave propagated through the device inthe direction of higher attenuation. Forward loss is the attenuationsuffered by a wave propagated in the direction of lower attenuation.Return loss is the ratio between the amplitude of awave impressed uponthe device in either direction and the amplitude of the resultantreflected wave returned by the device in the direction from which theoriginal wave is impressed. Each of these values of loss may beexpressed in decibels in well known manner.

A feature of the invention is the use of two gyromagnetic elements onopposite sides of the wave guide and the application of externalmagnetizing fields of opposite senses to the respective gyromagneticelements with the result that the two elements act in mutually aidingrelationship. Applicants have found that the variation of the fieldpatterns for wave propagation in the forward and reverse directions iseven more favorable to a large isolating effect than is the case for asingle gyromagnetic element. 7

Certain features of the disclosed devices applicable equally well toeither single or double element isolators are claimed by S. Weisbaum assole inventor in his application Serial No. 503,678, filed April 25,1955, or by F. J. Sansalone and S. Weisbaum jointly in their applicationSerial No. 503,677, filed April 25, 1955, concurrently herewith.

In the drawings: 7

Fig. l is a perspective view of a single slab isolator showing theconstituent parts partially disassembled and with a portion broken away;

Fig. 2 is a cross-sectional view at the plane indicated by line 2-2 inFig. 1;

Fig. 3 is a diagram illustrating electric field intensities over a crosssection similar to that shown in Fig. 2;

Fig. 4 is a diagram showing various regions of the surface of a ferriteelement characterized by different ranges of value of electric fieldintensity as a function of frequency;

Fig. 5 is a diagrammatic showing of a configuration of resistivecoatings on a ferrite surface;

Fig. 6 is a graph of values of reverse loss and return loss measuredover an extended frequency range in a device like that shown in Fig. 1;

Fig. 7 is a graph of values of forward loss measured over the samefrequency range in the same device represented by the graph in Fig. 6;

Fig. 8 is a cross-sectional view of a two-element embodiment illustratedin similar fashion to the view of a single-element embodiment as shownin Fig. 2; and

Fig. 9 is a diagram for the two-element case corresponding to thediagram of Fig. 3 for the single-element case.

In Fig. l is shown an exterior view of a single-element devicecomprising a length of hollow-pipe wave guide 10 of rectangular crosssection with terminal flanges 11 and 12 provided for convenience inconnecting the wave guide 10 into a wave guide assembly. For clarity inthe drawing only one flange 11 is shown. A permanent magnet 13 ofgenerally U-shaped cross section and of length comparable with that ofthe wave guide 10 is shown enveloping a portion of the wave guide andsecured in position by clamps such as clamp 14.

In addition there is shown a holder 15 as of foamed polystyrene and asingle ferrite element or slab 16 with sprayed resistive coatings 17 and18 (Fig. 5) on one face thereof. The magnet 13 is provided with holepieces 19 and 20. The holder 15 contains a recess 21 (Fig. 2) accuratelypositioned to support the ferrite element 16 in a preferred locationwithin the wave guide 10. Close fitting of the element 16 in the recess21 may eliminate any need for a covering or spacing member between theelement 16 and the adjacent side wall of the wave guide 10. The magnet13 is so placed that it provides a suitable biasing flux through theelement 16. The element 16 in the embodiment shown is placed with theresistively coated face against the inner surface of the recess 21.

Fig. 2 shows a cross section of the device of Fig. 1. The longerv insidedimension of the rectangular cross section of the wave guide 10 isdesignated L and the smaller inside dimension S. The thickness of thecross section of the element 16 is designated 6 and its spacing from thenearest narrow side wall of the wave guide is designated 7. Thethickness of the resistive coating upon the face of the element 16 isdesignated t.

The presence of the ferrite element influences the field pattern ofelectromagnetic waves transmitted through the wave guide, the influenceresulting in different field patterns for the two opposed directions ofwave propagation. The ferrite element is given such a thickness and isso spaced from the wave guide wall as to produce for one direction ofwave propagation a wave pattern having a relatively very low value ofelectric component of the field intensity at all times in a planeparallel to the longitudinal axis of the wave guide 10 lying in theresistively coated face of the element 16. For wave propagation in theopposite direction, the electric component of the field intensity inthis plane is found to have a relatively high value, particularly whenthe ferrite thickncss and spacing and the resistive coating thicknessare interrelated with the free space wavelength in accordance with Toreduce wave reflections and eddies 'due to disturbance of the otherwiseuniform impedance of the wave guide caused by the presence of theferrite element, the height h of the ferrite is preferably made somewhatless than S, the height of the wave guide.

The impedance match between an air region and an air-ferrite-air regioncontaining the element 16 with space above and below the element as inFig. 2 is generally better than that between an air region and a pureferrite region where the element extends from top to bottom of the waveguide.

Fig. 3 shows in full line 60, for a ferrite element the full height S,the approximate variation of the electric component of field intensityalong the width of the wave guide in the presence of the ferrite elementfor the direction of wave propagation that results in the minimumattenuation. A full height ferrite element is illustrated becausecalculations of field pattern are available for that case.Theoretically, in the absence of an electric component of fieldintensity, no energy is absorbed in the resistive coatings. Practically,the field intensity does not disappear completely for the less than fullheight ferrite element or for the full height element and a minimum ofabsorption is observed in either case. The broken line 61 in the figureshows the approximate variation of the electric component of fieldintensity for the reverse direction of wave propagation. As in this casethere is an electric component of considerable intensity at the ferritesurface, the resistive coatings absorb a large amount of energy from thewave thereby causing a substantial attenuation of the wave, i. e., alarge reverse loss.

The sensitivity of the device to the thickness and place ment of theferrite element affects principally the minimum ratio which will befound to exist between reverse loss and forward loss at a particularfrequency.

To enable the device to operate over a relatively broad frequency band,the resistive coatings may be given a particular configuration which hasbeen found most favorable.

Various portions of the face bearing the resistive coatings are found toinfluence the energy absorption differently at frequencies above orbelow the frequency of best operation.

Fig. 4 shows the face of the ferrite element 16 divided into three kindsof regions designated A, B, and C, respectively. In a wave guide thatwas designed for operation in a frequency band extending from 5925 to6425 megacycles per second, the characteristic effects of the regionswere found to \be as follows.

Region A had no effect on the forward loss but gave maximum absorption,i. e. high reverse loss, at 5925 megacycles per second and little or noeffect on either forward loss or reverse loss at 6425 megacycles persecond.

Regions B had little or no effect on either forward loss or reverse lossat the lower frequency but gave maximum absorption at the higherfrequency with substantially no effect on forward loss.

Regions C had substantially no effect on forward loss and produced aslight increase in reverse loss at the lower frequency but increased theforward loss considerably at the higher frequency.

Accordingly, it is preferable to omit resistive coating in regions C.The presence of resistive coating in regions A and B is evidentlyconducive to broad band operation. Therefore, the coatings arepreferably confined substantially to regions A and B. It has been foundalso that it is advantageous to have a lower effective resistance inregion A than in regions B. These requirements are conveniently met byapplying two overlapping coatings.

Variations in field pattern in the vertical direction in Fig. 2 givingrise to various regions as shown in Fig. 4 are reasonably to be expectedinasmuch as the ferrite element does not extend from top to bottom ofthe wave guide. Hence the electric and magnetic properties vary somewhatin going from the air region to the ferrite region along the verticaldirection. This is in contradistinction to the case wherein the ferriteelement does extend from top to bottom of the wave guide, in which casethe electric and magnetic properties tend to be uniform along thevertical direction.

Fig. 5 shows a preferred scheme of overlapping coatings. One coating 17comprises a centrally located longitudinally extending stripapproximately half the length of the element 16. The other coating 18comprises a diagonally located strip approximately as long as thediagonal of the rectangular face of the element 16.

The arrangement of resistive coatings in the type of configuration shownin Fig. 5 embodies the joint inven tion of F. J. Sansalone and S.Weisbaum, claimed in their application Serial No. 503,677, filed April25, 1955, hereinabove cited.

Fig. 6 shows the variation of the reverse and return losses and Fig. 7the variation of the forward loss, each in decibels over the frequencyband from 5925 megacycles per second to 6425 megacycles per second forthe hereinabove mentioned embodiment that was built substantially inaccordance with Figs. 1 and 5 and successfully operated. The forwardloss is 0.15 decibel over the range from 5925 to 6300 megacycles persecond and rises to a maximum of 0.22 decibel at 6425 megacycles persecond. The reverse loss is substantially uniformly 30 decibels over theentire range. The return loss is at least 32.5 decibels over the samerange.

The value of the saturation magnetization of the ferrite selected isimportant for best operation, i. e. maximum loss ratio. In theliterature, the saturation mag- In the frenetization is commonlydesignated by 41rM quency band of 5925 megacycles per second to 6425megacycles per second, a value of 41rM equal'to 1700 gauss was found tothe optimum. Values as low as 1600 gauss and as high as 1800-gauss wereslightly inferior.

It is found that where a satisfactory isolator is designed for onefrequency band the design parameters may be scaled up or down to derivea satisfactory isolator for a difierent frequency band. In particular,the saturation magnetization of the ferrite should be one selected tohave a value in direct proportion to the change in the midbandfrequency. At the same time, both the height and the thickness of theferrite element should be changed in the inverse ratio of the mid-bandfrequencies. For example, an isolator which gives good performance in afrequency band centered at 6000 megacycles per second, may be used asthe basis for the design of an isolator which will give substantiallythe same performance in a frequency band centered at 12,000 megacyclesper second. For the latter band, the saturation magnetization should betwice that for the former band. The height and thickness of the ferriteelement should at the same time each be half as great as before. The newvalue of saturation magnetization may be obtained by selecting aspecifically different ferrite from many available such materials whichare known to present a fairly wide choice of values of saturationmagnetization.

The resistive coatings 17, 18 may be strips of carbon cated polyethylenematerial pasted or otherwise secured to the face of the ferritematerial. Alternatively, resistive material such as graphite may besprayed on the surface over areas limited as by a metal mask. In thearrangement of Fig. 5, two masks may be employed, one for thelongitudinal strip and one for the diagonal strip. A suitable spraycoating was found to have a specific resistivity of 96 ohms per squareand the overlapping of the two coats was found to produce a specificresistivity in the overlapping area of 72 ohms per square.

Fig. 8 shows a two-element or double slab embodiment according to thepresent invention, using one ferrite slab on each side of the center ofthe wave guide. In the figure, the ferrite elements 30 and 31, each ofthickness 6 are mounted at a distance 7 from the narrower wall of theWave guide. As the rotation of the magnetic vector of the traveling waveis in opposite senses in the two sides of the wave guide, it isnecessary in order to provide the same relationship between theexternally applied magnetizing field and the rotating magnetic componentof the field of the traveling wave on both sides of the center, that thedirection of the externally applied magnetizing field the in the reversesense through the respective ferrite elements. For this purpose magnets32, 33 of opposite polarity are provided. Each element 30,31 has one ormore resistive film strips 34 mounted on the side of the element nearerthe center of the wave guide.

In an embodiment of the double slab isolator that was made andsuccessfully operated, the inside dimensions of the wave guide wereL=0.9 inch S=0.4 inch The optimum displacement 'y of each ferriteelement from the nearest side wall was found to be 0.028 inch. Thethickness 6 of each ferrite element was 0.099 inch. Each element had aheight of 0.303 inch and a length of 5 inches. The resistive coatingswere sprayed graphite strips each one-sixteenth inch wide, thelongitudinal strip being 2.75 inches long and the diagonal strip being4.5 inches long. The estimated thickness of the resistive coating wasone to two mils. The metallic proportions in the ferrite were Ni Cu ZnMn Fe having a saturation magnetization 41rM of approximately 3600gauss. The applied external magnetizing field supplied by each magnet32, 33 was approximately 1045 gauss. The isolator was designed for thefrequency band from 10,700 to 11,700 megacycles per second.

Applicants discovered the fact that the reverse loss resulting from-thejuxtaposition of two ferrite elements in a single section of wave guideis materially more than twice the reverse loss for an otherwiseidentical device containing but one ferrite element as in thearrangement of Figs. 1-3. For example, where the single slab isolatorgave a measured minimum reverse loss of 26 decibels, the correspondingdouble slab isolator gave a measured minimum reverse loss of 64 decibelsat 10,700 megacycles per second and a reverse loss of over 69 decibelsfrom 10,800 to 11,700 megacycles per second. In each case the forwardloss was not over one decibel.

The greater than expected value of reverse loss is believed to beattributable to a more favorable wave pattern developed in thesymmetrical arrangement of the double slab, isolator. The distributionsof the electric field for full height ferrite elements are believed tobe qualitatively of the type shown in Fig. 9, the solid curve 62 beingfor propagation in the forward direction and the dotted curve 63 for thereverse direction.

It is to be understood that the above-described arrangements areillustrative of the principles of the invention. Numerous otherarrangements may be devised by those skilled in the art withoutdeparting from the spirit and scope of the invention.

What is claimed is:

1. In a device for producing nonreciprocal wave transmission bydisplacing the electromagnetic field distribution of wave energy withgyromagnetic material to provide an electric field intensitydifferential in wave energy being transmitted in opposite directionsthrough said device and including material of electrical characteristicdifferent fromsaid gyromagnetic element for converting said fieldintensity differential into a nonreciprocal attenuation, a wave guide,two elements of said gyromagnetic material mounted on opposite sides ofand spaced away from the wave guide, each said element having a coatingof said different material attached to a portion of the surface thereof,and externally applied means magnetizing the respective gyromagneticelements in opposite senses.

2. In a device for producing nonreciprocal Wave transmission bydisplacing the electromagnetic field distribution of wave energy withgyromagnetic material to provide an electric field intensitydifierential in energy being transmitted in opposite directions throughsaiddevice and including resistive material having a loss characteristicdifferent from said gyrornagnetic element for converting said fieldintensity differential into a nonreciprocal attenuation, a wave guide,two elements of said gyromagnetic material mounted within the wave guideon opposite sides thereof and spaced from the walls thereof, each saidgyromagnetic element supporting said resistive material in the form ofan individual resistive element thereon, and means to magnetize the saidgyromagnetic ele ments in opposite senses.

3. In a device for producing nonreciprocal wave transmission bydisplacing the electromagnetic field distribution of wave energy withgyromagnetic material to provide an electric field intensitydifferential in wave energy being transmitted in opposite directionsthrough said device and including material of electrical characteristicdifferent from said gyromagnetic element for converting said fieldintensity differential into a nonreciprocal attenuation, a wave guide,said gyromagnetic two slabs of said gyromagnetic material mounted withinthe wave guide on opposite sides thereof and spaced from the wallsthereof, each said slab having a coating of said diiferent material onone side thereof, and externally applied magnetic means to magneticallypolarize the slabs in opposite senses.

4. In a devce for producing nonreciprocal wave transmission bydisplacing the electromagnetic field distribution of wave energy withgyromagnetic material to provide an electric field intensitydifferential in wave energy being transmitted in opposite directionsthrough said device and including material of electrical characteristicdifferent from said gyromagnetic element for converting said fieldintensity differential into a nonreciprocal attenuation, a rectangularwave guide, two elements of said gyromagnetic material extendinglongitudinally Within said guide and spaced away from the narrower wallsthereof, a strip of said material mounted on each one of said elements,and means for magnetically biasing said elements in opposite senses.

5. In a device for producing nonreciprocal wave transmission bydisplacing the electromagnetic field distribution of wave energy withgylfqmagnetic material to provide an. elec ric field in en i y differ ntl n n y being transmitted in opposite directions through said device andincluding resistive material having a loss characteristic different.from said gyromagnetic element for converting said field intensitydifferential into a nonreciprocal attenuation, a section of rectangularwave guide, two elements of said gyromagnetic material positioned withinsaid guide parallel to the narrower walls thereof, a nondissipativedielectric medium extending between each of said elements and theadjacent one of said narrower walls, a strip of said resistive materialmounted on the inner face of each of said elements, and means forestablishing a magnetic field in said elements in opposite senses.

6. In combination, a conductively bounded rectangular wave guide forelectromagnetic wave energy, two flattened vanes of oppositelymagnetically polarized gyromagnctic material located within said guidewith the parallel faces thereof extending parallel to the narrower wallsof said guide, each of said vanes being spaced away from said narrowerwalls to produce substantially a zero intensity of the electric field ofenergy propagating in one direction within said guide along the plane ofthe face of each of said vanes farthest removed from the closest one ofsaid narrower walls, and resistive material located upon each of saidfaces substantially in said planes.

References Cited in the file of this patent UNITED STATES PATENTS2,197,123 King Apr. 16, 1940 2,719,274 Luhrs Sept. 27, 1955 2,731,603Weber Ian. 17, 1956 2,760,171 King Aug. 21, 1956 2,776,412 Sparling Jan.1, 1957 FOREIGN PATENTS 674,874 Great Britain July 2, 1952 OTHERREFERENCES Fox et al.: Behavior and Application of Ferrites, BellSystem, Technical Journal, vol. 34, No. 1, January 1955, pp. 5-104.

